1. Field of the Invention
The present invention relates to a camera having a lens shutter mechanism, and more particularly, to a camera having a lens shutter mechanism capable of changing aperture speeds.
2. Description of the Related Art
Various shutter lens mechanisms capable of changing shutter speeds that are synonymous of aperture speeds have been known in the past. For example, a technique using a governor is an example of a technique of changing shutter opening speeds.
Japanese Patent Laid-Open No. 55-96925 has disclosed a technological means for opening or closing sectors using a magnetic force means such as a motor. The voltage to be applied to a conductor such as a coil lying in a magnetic field or the duty ratio of pulsating voltage produced by a pulse oscillator is varied in order to change speeds of opening and closing sectors.
The means disclosed in the Japanese Patent Laid-Open No. 55-96925 is advantageous to a programmable shutter actuated at a low shutter speed. However, there are difficulties in realizing a high-speed shutter because when a magnetic force is used to open and close sectors, a high torque is hardly yielded. For realizing a high-speed shutter, springs must be employed to utilize the constraining forces.
Japanese Patent Laid-Open No. 3-116032 has disclosed another example, wherein the constraining force exerted by a first spring resting on a member for driving sectors is utilized to open sectors at a low speed, and the constraining force exerted by another spring or a second spring resting on the member as well as the constraining force exerted by the first spring is utilized to open the sectors at a high speed. In this mechanism, the switching of the springs is achieved by rotating a ring mounted in a lens frame. Release or closing of shutter blades is triggered by an actuator.
The Japanese Patent Laid-Open No. 3-116032 describes a structure in which one spring is used to constrain a single member to move in a direction permitting low-speed opening and another spring is added for a movement in a direction permitting high-speed opening. This structure is very complex, resulting in increases in the number of assembling processes and in cost. Moreover, it is hard to manage two spring forces. This results in unstable sector opening speeds. Furthermore, since a setting ring mounted in a lens frame is used to change sector opening speeds, when exposure modes are changed, not only a camera body but also a lens frame must be manipulated for mode selection. Thus, complex and time-consuming manipulations are needed for mode selection.
As for location of a lens shutter mechanism in a camera, various proposals have been made in the past. For example, Japanese Patent Laid-Open No. 48-29427 describes a lens shutter mechanism that is arranged along an arc on an outer circumference of a lens and thus stowed in a lens frame. This kind of arrangement of a lens shutter mechanism is generally adopted.
In the Japanese Patent Laid-Open No. 48-29427, the entire shutter mechanism is disposed in a lens frame. This poses a problem that there arises a difficulty in realizing a small-sized lens frame. Eventually, a camera body becomes larger in size.
Another example of the lens shutter mechanism is described in Japanese Examined Patent Publication No. 47-28939, wherein a shutter mechanism is divided into two portions that are respectively arranged inside and outside of a lens frame. A major portion of the shutter mechanism is disposed outside the lens frame. The major portion is deconcentrated two-dimensionally over a wide range of a front surface of a camera body.
According to the Japanese Examined Patent Publication No. 47-28939, the shutter mechanism is deconcentrated over the front surface of the camera body. This means that the shutter mechanism lies in front of a spool chamber or a Patrone chamber. The thickness of the camera body therefore increases by the thickness of the shutter mechanism.
Yet another example has been disclosed in Japanese Patent Laid-Open No. 54-24621, wherein a base locked in a lens frame is bent in the form of a letter L, and part of a shutter mechanism is disposed two-dimensionally in a lower area of a camera body. In this case, members other than the lens frame are deconcentrated two-dimensionally along an outer circumference of the lens frame or in the lower area of the camera body.
The Japanese Patent Laid-Open No. 54-24621 has a problem that since a film wind gear array lies in the lower area of a camera body, when the shutter mechanism is disposed in the lower area, the camera body cannot be designed compactly. Moreover, the base in which the shutter mechanism is mounted is locked in the lens frame. This structure of a shutter mechanism is therefore unsuitable for a type of camera in which when unused, a lens frame is collapsed.
Sectors forming a lens shutter pass a beam of light reflected from an object only for the purpose of photography. The sectors must not be opened accidentally on any other occasion except for photography. In a generally known mechanism for preventing the sectors from opening incorrectly, a spring is attached to sectors or a member for driving sectors such as a drive ring. The spring force is used to constrain the sectors to remain closed.
An example of this kind of mechanism is described in Japanese Unexamined Utility Model Publication No. 2-55224 relating to a camera in which lenses are reset to a position outside of an effective focusing range every after photographing operation. That is to say, a boss or any other member for immobilizing closed sectors protrudes from a position, to which lenses are reset, on a camera body, thus preventing incorrect opening of the sectors. The member for immobilizing the sectors after the sectors close is designed to immobilize the sectors directly or immobilize a switch lever.
According to the Japanese Unexamined Utility Model Publication No. 2-55224, when the boss attached to the camera body is used to directly immobilize the sectors, since the sectors are formed with very thin plate members, the sectors may be damaged. The sectors open and close in a limited space at a high speed. Therefore, when the sectors are damaged, it may lead to malfunction. Moreover, this structure makes it necessary to include an auxiliary boss as an immobilizing boss. This is disadvantageous cost-wise. Besides, an extra space is required for attaching a boss. Furthermore, a shutter mechanism is entirely incorporated in a lens frame, making it difficult to realize a small-sized camera.
Yet another example is described in Japanese Patent Laid-Open No. 4-86635, wherein a lens frame is reset to a predetermined reset position every after photography, and a pin protruding from the the reset position on the lens frame is coupled with a switch lever in order to prevent incorrect opening of sectors. The last-mentioned patent publication relates to a type of camera in which when a camera is unused, a lens frame collapses to enter a camera body. Herein, when the camera is used, a constraining force exerted by a spring is used to constrain a shutter means to close. When the lens frame collapses, the pin protruding from the camera body toward an object immobilizes the switch lever so as to prevent incorrect opening of sectors.
According to the Japanese Patent Laid-Open No. 4-86635, a spring is used to constrain sectors to close and to thus prevent the sectors from opening incorrectly. Herein, a force for opening the sectors must be strong enough to cancel out the force for constraining the sectors to close. When a spring force is employed to open and close sectors, there arises a difficulty in controlling the balance between a constraining force of a spring for preventing incorrect opening of sectors and a constraining force of a spring for opening the sectors. In particular, when the sectors are opened at a low speed, the difficulty in controlling the balance becomes marked. Even a minor mechanical difference leads to unstableness in a sector opening speed. As described in the Japanese Unexamined Utility Model Publication No. 2-55224, when a shutter mechanism is designed to be stowed entirely in a lens frame, it becomes hard to design the lens frame compactly. Consequently, a camera body itself becomes large in size. According to the structure in which a pin is fitted in order to prevent incorrect opening of sectors, the number of parts increases and an extra space becomes necessary.
For example, Japanese Unexamined Utility Model Publication No. 59-27532 relates to a camera shutter comprising a first sector drive ring member that, when rotated by means of a drive spring force, allows sectors to open, a second sector drive ring member that, when rotated by means of a drive spring force in the same direction in which the first sector drive ring member rotates, allows the sectors to close, and first and second electromagnetic devices for rotating the first and second sector drive ring members resistively to the drive spring forces, attracting and retaining the first and second sector drive ring members at set positions allowing the sectors to remain closed, and freeing -the first and second sector drive ring members when actuated. While the first sector drive ring member alone is rotating by means of the drive spring force, the sectors open. When the second sector drive ring member rotates by means of the drive spring force, the sectors close. Disclosed in the last-mentioned utility model publication is a bound prevention device for a camera shutter, comprising a spindle resting on a first setting member for pushing the sector drive ring members to the set positions, a bound prevention lever for immobilizing the sector drive ring members when the camera shutter is at a halt, and a release member that rests on a second setting member, which drives the first setting member so that the first setting member pushes the sector drive ring members to the set positions, as part of the second setting member, and that, after the pushing is started, interlocks with the bound prevention lever so as to free the sector drive ring members. According to a technological means described in the last-mentioned utility model publication, a close ring for closing the sectors is caught between a shutter board and the bound prevention lever resting on the shutter board. Thus, the sectors are prevented from reopening. Release of the bound prevention lever for setting is achieved using a cam.
However, the Japanese Unexamined Utility Model Publication No. 59-27532 poses a problem in that the mechanism is complex and large in size because a special structure such as the cam is needed to release bound prevention after bound prevention is effected.